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A Big Gap Between Training and Development Error Suggests Overfitting

In this document-classification example, training error is 3% and development error is 14%, an 11-percentage-point generalization gap. If the training and development sets come from the same distribution, this large gap is evidence of high variance, or overfitting: the model performs much better on its training examples than on unseen examples. If the development set has a different distribution, these two error rates alone cannot distinguish variance from data mismatch; that diagnosis requires comparison with a training-dev set drawn from the training distribution.

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Updated 2026-08-30

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